The magnetic domain structure of melt-spun giant magnetoresistant Au71.6Co28.4 samples in as-prepared and annealed states is determined by means of Lorentz microscopy using the JEOL-1000 atomic resolution microscope (ARM) in a modified diffraction mode. The interpretation of the domain structure is done by comparing magnetic contrasts observed in Fresnel mode with simulated contrasts using a geometrical algorithm based on the Lorentz equation. Cobalt precipitates 50 to 100 nm in diameter are found in a single domain state with a curling-like magnetization, that reduces their magnetic stray fields. Larger precipitates (approximate to 200 nm) are multidomain particles.